Each of these documents, and in general any document which represents currency or has an intrinsic value, requires particular solutions aimed at preventing its duplication and counterfeiting.
An evident and well-known example of products which require one or more security elements is constituted by banknotes. As is known, each banknote is in fact provided with characteristic elements, such as watermarked paper which shows symbols or figures which are impressed so that they are visible only if viewed by transmitted light, a silver thread, holograms or others.
Duplicating a banknote, i.e., producing false banknotes, therefore requires knowledge and exact duplication of the methods with which these security elements are provided. Since the main requirement of these security elements remains that they must not be easy to reproduce, very complex production methods have been devised in the course of time which entail an elaborate sequence of steps and the use of elements having particular chemical and physical characteristics. These methods allow to obtain effects, generally optical ones, which are very difficult to reproduce exactly and therefore allow to distinguish a genuine banknote from a counterfeit.
Known types of security elements comprise, for example, security threads, stripes or patches. By way of non-limiting example, the security element according to the invention and the method for obtaining such security element will be described hereinafter in one of its possible applications, i.e., in the field of banknotes. However, it is straightforward for the person skilled in the art to understand that the same considerations apply to any document having similar characteristics.
When a banknote has been circulating for some time, the decision may be taken to replace it with new banknotes of a different type. Gradually, the old banknotes are withdrawn from circulation and replaced by the new banknote. The issue of new banknotes can be decided also because it is deemed necessary to increase the level of security by replacing the banknotes with others of a new kind provided with more sophisticated security elements, produced with techniques which are new, original and difficult to duplicate. In this manner, even if ill-intentioned individuals have succeeded in understanding how to reproduce partially the security elements of the banknotes, the knowledge and the degree of experience acquired by the counterfeiter would not be easily transferable to the new banknotes, provided with the new and more sophisticated security elements.
Currently, most banknotes issued worldwide are provided with a security thread, which is inserted in the paper according to two different techniques, known respectively as “total embedded” and “windows”.
With the first technique, known as “total embedded”, the thread is inserted completely within the thickness of the paper and therefore the security thread is completely surrounded, on both faces, i.e., recto and verso, by the paper.
With the second technique, known as “windows”, the security thread is inserted in the banknote so that it is completely covered by the paper on one face of the banknote whereas on the other side there are regions of the thread which are exposed and are alternated with covered regions of thread with a planned alternation.
Currently, some security threads can have, when viewed, a coloring which can vary depending on the angle of incidence of the light and/or on the type of light that strikes them.
This characteristic is known as “color shift”.
The types of thread currently in use may have a color shift provided by means of a so-called thin-film technique. This thin-film technique consists in vaporizing, on the full surface and in vacuum, elements such as magnesium fluoride associated with chromium or aluminum in an extremely low thickness on a polyester substrate. Products manufactured with this technique allow, depending on the amount of material that is vaporized, to view a reflective surface in two different colors depending on the inclination with which light strikes them.
An alternative manufacturing method that is currently used entails providing the color shift effect by means of a pigment-based technique. In this technique, a polyester substrate is printed with inks which contain pigments, liquid crystals, et cetera, which have the property of reflecting colors in two distinct ranges depending on the inclination with which light strikes them. In order to achieve this effect, the pigment must be printed on backgrounds which have very dark colors, typically dark gray or black.
The first production technique described above, known as “thin film”, provides full-surface elements from which material is then removed (demetallization) in some areas, particularly at the regions where an etching is to be provided, by way of known kinds of techniques, based on the use of waxes or resist. These areas without material therefore leave graphic markings, for example letters or numerals, depending on where the material is removed. In this manner, it is possible to provide any graphic marking one wishes, as a positive or as a negative.
The second production technique described above, known as “pigment-based technique”, instead entails printing the ink on the full surface on a black background which has already been printed selectively, again as a positive or as a negative.
The production methods can be summarized briefly as follows. As regards the thin-film technique, it is possible to print wax-based graphic markings before the vacuum vaporization step, subsequently softening these waxes so that their removal also removes the color-shifting material. As an alternative, it is possible to print in regions above the aluminum vaporization or color-shifting materials a resist which allows to preserve the color-shifting material from acid or basic elements, depending on the process. These acid or basic elements corrode, and therefore remove, the parts of material which are not covered by the resist.
As regards the pigment-based technique, systems are currently used which print the colors sequentially, depositing the image that is to be obtained; for example, texts are printed in negative by using a dark color with the first printing section, followed by the full-surface color-shifting color with the second section.
It is known that in security threads inserted in banknotes, the side that remains inserted within the paper of the banknote, i.e., the side which lies opposite the windows described earlier if the security thread is inserted with the window technique, must be very light in color, for example white or straw yellow, or highly reflective, such as aluminum. This is needed in order to obtain the well-known effect of making the thread inserted in the banknote practically invisible when viewed by reflected light and by viewing the face in which it is fully inserted within the paper of the banknote, obtaining at the same time maximum opacity when the same face of the banknote is viewed by transmitted light, i.e., against the light. With the techniques known up to now, it is not possible to print, for example, the following sequence of colors: with a first section, an aluminum gray color which has texts in negative; with a second section, a black color which has the same texts in negative; and then another pair of additional colors with a third section and a fourth section (or, worse still, with a second pass), which have the same texts in negative exactly superimposed on the first two. By viewing by transmitted light from the recto and from the verso the texts printed in negative, the colors printed with the first section and the colors of the third and fourth sections must not allow to see that inside the multilayer element there is the black color printed with the second section.